Generally, in the MOS type semiconductor device, LOCOS (local oxidization of silicon) isolation has been used as element isolation for a long time, and STI (shallow trench isolation) has been come to use in recent years due to the miniaturization thereof.
In the CMOS solid-state imaging device as well, the STI has been used for element isolation. A CMOS solid-state imaging device includes a pixel region and peripheral circuitry which drives the pixel and performs signal processing, and a miniaturization technology of the peripheral circuitry is also employed in the pixel region. In element isolation of pixels in the recent miniaturized CMOS solid-state imaging device as well, the same STI element isolation method as that for the periphery is used in general.
FIG. 9 schematically shows a structure seen from the upper surface of the relevant portion of a pixel region in a CMOS solid-state imaging device; and FIG. 10 shows the structure in cross-section of a photodiode (photoelectric conversion portion) and a transistor of a pixel according to conventional STI isolation. Note that FIG. 10 corresponds to the section of A-A line of FIG. 9.
In a CMOS solid-state imaging device, unit pixels 2 [2A, 2B, 2C and 2D] are formed, each of which includes a photodiode PD constituting a photoelectric conversion portion and a plurality of MOS transistors; and a pixel region 3 is formed of a plurality of unit pixels 2 being arranged in the form of a matrix. The unit pixel 2 includes, for example, one photodiode PD and four CMOS transistors, namely a readout transistor, a reset transistor, a selection transistor and an amplification transistor. In FIG. 9, only the photodiode PD, and a readout transistor Tr1, a reset transistor Tr2 and an amplification transistor Tr3 that are connected to the photodiode are shown as the unit pixel 2, and a selection transistor is omitted in order to simplify explanations. The readout transistor is formed of a charge-accumulation region of the photodiode PD, a source drain region 4 to be a floating diffusion (FD), and a transfer gate electrode 5 formed with a gate insulating layer in between. The reset transistor Tr2 is formed of a pair of source drain regions 4 and 6, and a reset gate electrode 7 formed with a gate insulating layer in between. The amplification transistor Tr3 is formed of source drain regions 6 and 8, and a gate electrode formed with a gate insulating layer in between. A vertical signal line 10 is connected to one source drain region 8 of the readout transistor Tr1 of each of the pixels vertically aligned, and a power line which supplies power supply voltage Vdd is connected to one source drain region 6 of the reset transistor Tr2. An element isolation region 8 separates the pixels 2 [2A, 2B, 2C and 2D].
In a CMOS solid-state imaging device 11, in accordance with conventional STI isolation, as shown in the cross-sectional structure of FIG. 10, a p-type semiconductor well region 13 is formed in an n-type silicon semiconductor substrate 12, a trench 14 is formed in the p-type semiconductor well region 13 and a silicon oxide film 15 is buried in the trench 14, and an element isolation region separating adjacent pixels 2, namely an STI region 81 (which corresponds to the element isolation region 8 in FIG. 9), is thus formed, for example. By the STI region 81, a photodiode PD of the pixel 2B on one side and a photodiode PD of the pixel 2A adjacent thereto are separated, and also a floating diffusion (FD) of a readout transistor Tr1 of the pixel 2B on one side and a source drain region 8 of an amplification transistor Tr3 of the pixel 2A on the other side are separated.
A photodiode PD is formed having an HAD (Hall Accumulated Diode) structure including what is called an n-type substrate 12, a p-type semiconductor well region 13, an n-type charge-accumulation region 18, and a p+ accumulation layer 19 at the interface between the n-type charge-accumulation region 18 and an insulating layer 20 on the surface side thereof. A readout transistor Tr1 includes a transfer gate electrode 5 formed between the n-type charge-accumulation region 18 of the photodiode PD and a source drain region 4 to be a floating diffusion (FD) with a gate insulating layer 20 in between. On the other hand, in the STI region 81, a p+ semiconductor region 17 for restraining dark current and white spots is formed at the interface between the silicon oxide film 15 deeply buried and the n-type charge-accumulation region 18 and also at the interface between the silicon oxide film 15 deeply buried and the p-type semiconductor well region 13.
On the other hand, a technology of an element isolation portion is proposed, in which in an MOS solid-state imaging device, an element isolation portion is formed of an insulating layer made on a semiconductor substrate so as not to corrode the semiconductor substrate.
There are mainly two problems with respect to a CMOS solid-state imaging device using the above-described STI element isolation method as a pixel region isolation technology.
Since the deep trench 14 is formed in the silicon substrates (12 and 13) and the silicon oxide film 15 is buried therein to form an element isolation region 81, the STI element isolation method is excellent in forming a miniaturized element isolation region. However, due to a difference in thermal expansion coefficient between the silicon oxide film 15 deeply buried and the silicon substrates (12 and 13) and other factors, there is a first problem in which pixel defects caused by thermal stress easily occur. Therefore, the form of STI is designed to be a tapered-shape and other devices are contrived. However, to make the form of STI tapered results in narrowing the region of photodiodes PD, leading to reduction in the saturation signal amount and sensitivity.
A second problem is the existence of the p+ semiconductor region 17 at the interface between the silicon oxide film 15 within the trench 14 and the photodiode PD. In the photodiode PD, that is, an HAD sensor, the n-type charge-accumulation region 18 is depleted, however, a boundary with the silicon oxide film 15 needs to be covered with the p+ semiconductor region 17 made of a diffusion layer, in order to restrain dark current and white spots caused by the occurrence of a small number of current carriers from an interface level. Therefore, the interface of the silicon oxide film 15 of the STI region 81 needs to be covered with the p+ semiconductor region 17 having a similar concentration to the p+ accumulation layer 19 of the HAD sensor. To three-dimensionally cover the portion in the depth direction with a sufficient concentration is not easy from the viewpoint of production thereof. For example, it is difficult to introduce a p-type impurity to form the p+ semiconductor region 17 into the trench side surface of the STI region 81 by portion of ion implantation. Specifically, since an impurity is introduced into a narrow place (trench side surface) by portion of diagonal ion implantation, the ion implantation is difficult to be performed. Further, since the p+ semiconductor region 17 is introduced in an earlier process, there is a problem in which the p+ semiconductor region 17 enlarges a great deal toward the side of a photosensor (photodiode PD) due to thermal diffusion, narrowing the photosensor. Even if reduction in thermal treatment is possible, there is still a fundamental problem in which the region of the n-type charge-accumulation region 18 of the photosensor decreases as shown in FIG. 10 and so the saturation signal amount decreases, for the reason that the trench side of the STI region 81 needs to be covered with the concentrated p+ semiconductor region 17 in principle.
With element isolation by the STI region 81 based on a conventional miniaturization technology, minute isolation can be obtained as an isolation method for a simple MOS transistor; however, with respect to the isolation between pixels of the CMOS solid-state imaging device, there remain the above-described two problems, causing deterioration in images and making it difficult for miniaturized pixels to be manufactured.